(1) Field of the Invention
The present invention relates to a reactor that has gas perform a catalytic reaction with exothermic reaction, and especially relates to a CO remover used for a fuel-cell generator system.
(2) Related Art
Generally speaking, in a fuel-cell generator system, an electrochemical reaction of hydrogen-rich gas that is supplied to the fuel electrode of the fuel cell and air that is supplied to the air electrode generates electricity.
This hydrogen-rich gas is obtained by steam reforming of the mixture of water vapor and fuel by a reformer 101 as shown in FIG. 1. The fuel is alcohol such as methanol and light hydrocarbon such as natural gas and naphtha, which is easily available and is not expensive.
During the steam reforming reaction in the reformer 101, a high temperature is applied to a catalyst bed for reforming, and hydrogen is generated and carbon monoxide is generated as by-products.
For the fuel electrode of a fuel cell 104, catalyst such as platinum is used, however, carbon monoxide deteriorates the catalyst to lower the power generation performance. In order to prevent the deterioration, a CO shift converter 102 is disposed downstream from the reformer 101 in many fuel-cell generator system to transform carbon monoxide using water vapor as described below so that carbon monoxide with a lower concentration is supplied to the fuel cell 104.CO+H2O→CO2+H2 
Note that the CO shift converter 102 decreases the concentration of carbon monoxide only to around one % when the S/C (Steam by Carbon) in the steam reformer 101 is 2.5, for instance. In the case of a PEFC (Polymer Electrolyte Fuel Cell), which operates at a relatively low temperature, the concentration of the carbon monoxide in reformed gas needs to be lowered since the electrode catalyst is more likely to deteriorate.
For instance, the carbon monoxide concentration in reformed gas is lowered according to the fuel-cell generator system disclosed in Japanese Patent Laying-Open Publication No 8-100184. In the fuel-cell generator system, a CO remover 103 is disposed to add a small amount of air to reformed gas. Then, the reformed gas is passed through a selective oxidative catalyst bed to eliminate carbon monoxide by selectively oxidizing the reformed gas as described in a reaction formula below, and is supplied to the fuel cell 104.2CO+O2→2CO2 
In the CO remover, it is required to keep proper selectivity, i.e., to maximize the combustion of carbon monoxide with minimizing the combustion of hydrogen. For this purpose, it is important to keep the temperature of the selective oxidative catalyst bed in a proper range. Although different in kind of catalyst, the well-known proper range of temperature is from 140 to 190° C. for a ruthenium catalyst, for instance. When the temperature of the selective oxidative catalyst bed is higher than this range of temperature, the proper selectivity in oxidation is not kept. On the other hand, when the temperature is lower than this range, the combustion of carbon monoxide is not effectively performed.
In the selective oxidative catalyst bed, heat is generated by a selective oxidative reaction of gas. In order to keep the temperature of the oxidative catalyst bed in the range, the oxidative catalyst bed is cooled during the operation of the CO remover.
Note that, in such a CO remover, the reaction of reformed gas and air tend to be accelerated in the selective oxidative catalyst bed around the entrance of the CO remover where reformed gas and air come into first contact with the selective oxidative catalyst bed. As a result, the temperature around the entrance tend to be high and almost oxygen tend to be consumed here. When oxygen is consumed on the entrance side, the exit side is short of oxygen and a methane formation reaction tends to be caused as a side reaction.
As a result, in order to prevent side reactions and obtain high CO selectivity in the CO remover, it is required to keep the temperature of the selective oxidative catalyst bed in the proper temperature range and to control the selective oxidative reaction so that the selective oxidative reaction is performed evenly from the entrance side to the exit side of the CO remover in the selective oxidative catalyst bed.
It seems that precise control of air supply and cooling in each part of the selective oxidative catalyst bed easily realize such control. It is difficult, however, to precisely control the temperature in the CO remover that is simple in structure.
For instance, a well-known CO remover that is simple in structure has the structure described below. The CO remover is equipped with a selective oxidative catalytic device in which a cylindrical tube is filled with selective oxidative catalyst. In the CO remover, reformed gas and air are mixed and injected into the entrance of the cylindrical tube, and cooling water to cool the selective oxidative catalyst bed is supplied around the cylindrical tube to control the temperature of the selective oxidative catalyst bed. In this case, part of the selective oxidative catalyst bed that is nearer to the inner surface of the cylindrical tube (this part of the selective oxidative catalyst bed is called “peripheral part” in this specification) is closer to the cooling water, so that the temperature of the peripheral part of the selective oxidative catalyst bed tends to be relatively low and the temperature of part of the selective oxidative catalyst bed that is further from the inner surface of the cylindrical tube (this part of the selective oxidative catalyst bed is called “central part” in this specification) tends to be relatively high. As a result, even if the cooling is controlled so that the central part in the selective oxidative catalyst bed has a proper temperature, gas passing through the peripheral part has a temperature that is lower than the proper one, so that the reaction is not performed well and the CO selectivity is low. On the other hand, when it is controlled so that the peripheral part of the selective oxidative catalyst bed has a proper one, the central part tends to have a too high temperature and oxygen tends to be consumed around the entrance.
In order to solve the problems, the CO remover disclosed in Japanese Patent Laying-Open Publication No 8-47621 is equipped with first, second, and third reactors in this order from the upstream of the fuel gas. In the CO remover, air is separately supplied to each of the reactors, or the catalyst filling density of the selective oxidative catalyst bed is controlled so that the density is relatively low on the entrance side. This structure is effective to have the reaction performed evenly from the entrance side to the exit side and to obtain proper temperatures of the selective oxidative catalyst bed. In this case, however, it is required to provide a mechanism for allocating air or to control the filling density of the catalyst, so that the CO remover is not simple in structure.
Note that the problems described above are shared by not only a CO remover but also a reactor that supplies gas to a catalytic reaction device and has the gas undergo a reaction with exothermic reaction by having the gas pass through the catalytic reaction device.